1. Field of the Invention
The present invention relates to an analog finite impulse response filter and to a method of filtering an input signal using an analog finite impulse response filter.
2. Discussion of the Related Art
FIG. 1 illustrates an analog finite impulse response filter (analog FIR) 100. Such a filter is for example described in IEEE publication titled “A low-power 170-MHz discrete-time analog FIR filter”, Wang et al., Journal of Solid-State Circuits, vol. 33, No 3, March 1998.
Analog FIR 100 comprises an input node 102 for receiving an input voltage VIN, and a series of track-and-hold blocks 104, 106, 108 and 110, which sample the input signal at regular intervals offset with respect to each other. The outputs of blocks 104 to 110 are provided to a multiplexer 112, which connects these outputs to multipliers 114, 116, 118 and 120. Multipliers 114 to 120 multiply the signals from multiplexer 112 by respective coefficients C1 to Cn. The outputs of multipliers 114 to 120 are provided to an adder 122, which sums these outputs to provide an output voltage VOUT on line 124.
Such a filter has a number of drawbacks in terms of performance. In particular, it does not provide a clean cut-off, as high frequency components of the input signal can be allowed to pass through the filter, leading to undesirable noise.